Rippling of graphitic surfaces: a comparison between few-layer graphene and HOPG

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

The composition and structure of the ubiquitous hydrocarbon contamination on van der Waals materials

The behavior of single layer van der Waals (vdW) materials is profoundly influenced by the immediate atomic environment at their surface, a prime example being the myriad of emergent properties in artificial heterostructures. Equally significant are adsorbates deposited onto their surface from ambient. While vdW interfaces are well understood, our knowledge regarding atmospheric contamination is severely limited. Here we show that the common ambient contamination on the surface of: graphene, graphite, hBN and MoS₂ is composed of a self-organized molecular layer, which forms during a few days of ambient exposure. Using low-temperature STM measurements we image the atomic structure of this adlayer and in combination with infrared spectroscopy identify the contaminant molecules as normal alkanes with lengths of 20-26 carbon atoms. Through its ability to self-organize, the alkane layer displaces the manifold other airborne contaminant species, capping the surface of vdW materials and possibly dominating their interaction with the environment.

Here, the authors attribute the ambient surface contamination of van der Waals materials to a self-organized molecular layer of normal alkanes with lengths of 20-26 carbon atoms. The alkane adlayer displaces the manifold other airborne contaminant species, capping the surface of graphene, graphite, hBN and MoS₂.

Tailoring Peptide Self-Assembly and Formation of 2D Nanoribbons on Mica and HOPG Surface

Studying the interactions between biomolecules and material interfaces play a crucial role in the designing and synthesizing of functional bionanomaterials with tailored structure and function. Previously, a lot of studies were performed on the self-assembly of peptides in solution through internal and external stimulations, which mediated the creation of peptide nanostructures from zero-dimension to three-dimension. In this study, we demonstrate the self-assembly behavior of the GNNQQNY peptide on the surface of mica and highly oriented pyrolytic graphite through tailoring the self-assembly conditions. Various factors, such as the type of dissolvent, peptide concentration, pH value, and evaporation period on the formation of peptide nanofibers and nanoribbons with single- and bi-directional arrays are investigated. It is found that the creation of peptide nanoribbons on both mica and HOPG can be achieved effectively through adjusting and optimizing the experimental parameters. Based on the obtained results, the self-assembly and formation mechanisms of peptide nanoribbons on both material interfaces are discussed. It is expected that the findings obtained in this study will inspire the design of motif-specific peptides with high binding affinity towards materials and mediate the green synthesis of peptide-based bionanomaterials with unique function and application potential.

Origin of Ubiquitous Stripes at the Graphite-Water Interface

The investigation of solid-liquid interfaces is pivotal for understanding processes like wetting, corrosion, and mineral dissolution and growth. The graphite-water interface constitutes a prime example for studying the water structure at a seemingly hydrophobic surface. Surprisingly, in a large number of atomic force microscopy (AFM) experiments, well-ordered stripes have been observed at the graphite-water interface. Although many groups have reported on the observation of stripes at this interface, fundamental properties and, in particular, the origin of the stripes are still under debate. Proposed origins include contamination, interplanar stacking of graphene layers, formation of methanol-water nanostructures, and adsorption of nitrogen molecules. Especially, the latter interpretation has received considerable attention because of its potential impact on explaining the long-range nature of the hydrophobic interaction. In this study, we demonstrate that these stripes readily form when using standard plastic syringes to insert the water into the AFM instrument. In contrast, when clean glass syringes are used instead, no such stripes form even though nitrogen was present. We, therefore, conclude that contaminations from the plastic syringe rather than nitrogen constitute the origin of the stripes we observe. We provide high-resolution AFM data that reveal detailed structural insights into the arrangement of the stripes. The rich variability of our data suggests that the stripes might be composed of several different chemical species. Still, we cannot rule out that the stripes observed in the literature might originate from other sources; our study offers a rather straightforward explanation for the origin of the stripes. In the view of these results, we propose to carefully reconsider former assignments.